P.S. As far as I know, Id came into being due to experiments conducted on Fei. So the persona isn't the same age as him. What gets me though is how Id is anything other than a psychotic child, when he'd otherwise been supressed.spoiler]A crummy, half-finished, and otherwise warped game. Xenosaga ended up in a similar boat, but at least most of the cast, even supporting characters, remained relevant, likeable, and cool throughout the series. Xenogears on the other hand, you had a good first act, but when the party expanded beyond the first four or five characters, introductions would be made and that would be it for new additions.
The fact that the game was rushed and not completely finished does in NO way take away from it being the BEST RPG of ALL TIME. Opinion? Maybe. The opinion of nearly all critics and players who enjoyed the game? Yes. Generally, overwhelming opinions turn into fact before too long...
And it's not like we would all love a remake. There are plenty of flaws in the game however they do not out weigh the amazing plot, beautiful (main) character development, the music, the art, the fighting, the themes... Just about everything, to be completely honest.
But on topic.
One doesn't have persona's that are younger because they were "born" later. I mean they can PRETEND to be younger if that is what they want to do, however the age of the person is still the same. A persona is created to deal with a certain situation that the self does not want to or can't handle. Just because Id was developed during Fei's childhood he is not a baby for a while then a toddler then an adolescent so on and so forth. He's just what he is. A copping mechanism. Age is fairly irrelevant anyway.
